I guess. I think a true three party system would be if any or all of the parties could get into power. Everybody pretty much knows that the Lib Dems in the UK wouldn't have much chance of getting into power...which in my eyes is a shame. It tends to be Labour or the Conservatives and that's that. I don't know about the technical definition but to me a three party system would be three almost equal parties in size, stature and MP's.

I looked around myself actually and only found very complicated pieces of writing that I didn't understand. Either way I guess as long as the three parties are "credible" and recognised that's that. So I think after looking into it that we in Britain do have this. The third party is just smaller.